Depending on the quality of the stamp it could be worth quite a bit of money. Also, these stamps have issuing markings upon them which are indicative of the rarity of the issuing as well as other important attributes. The value of this stamp is best determined in person or by a dealer.
You could be looking at a twenty dollar valued stamp there. It all depends on the condition of the stamp and if it's been used. If it has been used then the value would drop substantially. Persevering these things is difficult as they require dry places to be stored to keep the adhesive from activating and drying up.
There are different types of Franklin Green one cent stamps actually, so it will be hard to get an answer without having any other info about the stamp. It could be worth pennies or much more. If it's in unused condition it increases the value.